[Bug c++/111284] [11/12/13/14 Regression] Some passing-by-value parameters are mishandled since GCC 9, affecting libstdc++'s constexpr std::string

["ppalka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111284

--- Comment #9 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #8)
> Created attachment 57648 [details]
> gcc14-pr111284.patch
> 
> So, I've tried to fix this by constexpr evaluating the arguments passed to
> PARM_DECLs with TREE_ADDRESSABLE types in the caller as lvalues rather than
> rvaluea and later, if we try to evaluate the PARM_DECL in the callee as lval,
> lookup the value and use that, if it is rval constexpr evaluate again as
> rvalue.
> There is a complication for qualified type, say if the argument is const in
> the callee and caller is passing reference to non-const, adjust_temp_type
> can't handle that when it isn't a rvalue.
Interesting, hopefully this fixes the std::string testcases in PR111258 and
related PRs?

And perhaps the following augmented testcase from this PR with a constexpr dtor
that checks valid():

void non_constant();

struct self_locator {
    self_locator() = default;
    constexpr self_locator(const self_locator&) noexcept : this_{this} {}
    constexpr self_locator& operator=(const self_locator&) noexcept { return
*this; }

    constexpr bool valid() const noexcept { return this_ == this; }
    constexpr ~self_locator() { if (!valid()) non_constant(); }

    self_locator *this_ = this;
};

constexpr bool demonstrator(self_locator x) noexcept
{
    return x.valid();
}

static_assert(demonstrator(self_locator{}), "");
static_assert([](self_locator x){ return x.valid(); }(self_locator{}), "");